Optical amplifier with pump light source control for Raman amplification

ABSTRACT

A Raman amplifier for amplifying a wavelength division multiplexed (WDM) light including signal lights wavelength division multiplexed together. The amplifier includes an optical amplifying medium and a controller. The optical amplifying medium uses Raman amplification to amplify the WDM light in accordance with multiplexed pump lights of different wavelengths traveling through the optical amplifying medium. The WDM light is amplified in a wavelength band divided into a plurality of individual wavelength bands. The controller controls power of each pump light based on a wavelength characteristic of gain generated in the optical amplifying medium in the individual wavelength bands.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on, and claims priority to, Japaneseapplication number 2000-255291, filed Aug. 25, 2000, in Japan, and whichis incorporated herein by reference.

[0002] This application is related to U.S. application Ser. No.09/531,015, filed Mar. 20, 2000, and which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] The present invention relates to a Raman amplifier for amplifyinga signal light in an optical communication system. More particularly,the present invention relates to a Raman amplifier for amplifyingwavelength division multiplexed signal lights.

[0005] 2. Description of the Related Art

[0006] Almost all optical amplifiers used in current opticalcommunication systems are rare-earth doped optical fiber amplifiers.Particularly, erbium (Er) doped optical fiber amplifiers (EDFA) arecommonly used.

[0007] Moreover, with wavelength division multiplexing (WDM) opticalcommunication systems, a plurality of signal lights at differentwavelengths are multiplexed together and then transmitted through asingle optical fiber. Since an EDFA has a relatively wide gain band, WDMoptical communication systems use EDFAs to amplify the multiplexedsignal lights. Therefore, with WDM optical communication systems usingEDFAs, the transmission capacity of an optical fiber can be greatlyincreased.

[0008] Such WDM optical communication systems using EDFAs are extremelycost effective, since they can be applied to previously laid opticalfiber transmission line to greatly increase the transmission capacity ofthe optical fiber transmission line. Moreover, an optical fibertransmission lines has virtually no limitation on bit rate since EDFAscan easily be upgraded in the future, as developments in opticalamplifier technology occur.

[0009] Transmission loss of an optical fiber transmission line is small(about 0.3 dB/km or less) in the wavelength band of 1450 nm to 1650 nm,but the practical amplifying wavelength band of an EDFA is in a range of1530 nm to 1610 nm. Thus, an EDFA is only effective for amplifyingsignals in a portion of the wavelength band of 1450 nm to 1650 nm.

[0010] In a WDM optical communication system, a predeterminedtransmission characteristic may be obtained by suppressing fluctuationof optical power among each channel to 1dB or less in each opticalrepeating stage because the upper limit of optical power is caused by anonlinear effect and the lower limit by a receiving signal-to-noiseratio (SNR).

[0011] Here, a transmission loss wavelength characteristic of thetransmission line and a dispersion compensation fiber or the likeforming the WDM optical communication system must be reduced.

[0012] In a WDM optical communication system, the wavelengthcharacteristic of transmission loss in a transmission line due to theinduced Raman scattering provides the maximum influence on thewavelength characteristic of the signal light.

[0013] A key component of current WDM transmission systems is an EDFAthat can amplify wavelength division multiplexed signal lights at thesame time. For further improvement, such as increase of transmissioncapacity and realization of ultra-long distance transmission, it wouldbe desirable to provide an optical amplifier which can amplify differentwavelength bands than a conventional EDFA, while also providing thefavorable characteristics of an EDFA.

[0014] In view of expanding the wavelength band of an optical amplifierto increase the transmission capacity of optical fibers, attention isbeing directed to a Raman amplifier.

[0015] A Raman amplifier can amplify the Stokes-shifted frequency thatis shifted as much as the Raman shift of the amplifying medium from thefrequency of a pump light. Therefore, a signal light can be amplified ata desired frequency with a pump light source producing a pump light of adesired wavelength.

SUMMARY OF THE INVENTION

[0016] Accordingly, it is an object of the present invention to providea Raman amplifier for use in a WDM optical communication system.

[0017] More specifically, it is an object of the present invention toprovide a control algorithm for a Raman amplifier using multiple pumplight wavelengths or pump sources to attain a flat wavelength band overa wide band range.

[0018] It is also an object of the present invention to provide acontrol algorithm for a Raman amplifier that allows the amplifier toeasily realize constant output power control, constant gain control andwavelength characteristic flattening control.

[0019] Additional objects and advantages of the invention will be setforth in part in the description which follows, and, in part, will beobvious from the description, or may be learned by practice of theinvention.

[0020] The foregoing objects of the present invention are achieved byproviding an optical amplifier including (a) an optical amplifyingmedium to Raman amplify a wavelength division multiplex (WDM) lightincluding signal lights wavelength division multiplexed together; (b)pump light sources generating pump lights of different wavelengths; (c)a first optical multiplexer multiplexing the pump lights together; (d) asecond optical multiplexer multiplexing the WDM light with themultiplexed pump lights; (e) a detector dividing the amplified WDM lightinto wavelength bands and detecting a power in each wavelength band; and(f) a pump light controller controlling power of each pump light basedon a wavelength characteristic of gain generated in the opticalamplifying medium for each wavelength bands, in accordance with thepowers detected, by the detector.

[0021] Objects of the present invention are also achieved by providingan optical amplifier including (a) an optical amplifying medium to Ramanamplify a wavelength division multiplex (WDM) light including signallights wavelength division multiplexed together; (b) pump light sourcesgenerating pump lights of different wavelengths; (c) a first opticalmultiplexer multiplexing the pump lights together; (d) a second opticalmultiplexer multiplexing the WDM light with the multiplexed pump lights;(e) an input detector detecting power of the WDM light before beingamplified by the optical amplifying medium; (f) an output detectordetecting power of the amplified WDM light; and (g) a pump lightcontroller controlling powers of the pump lights based on the powerdetected by the input detector and the power detected by the outputdetector.

[0022] Moreover, objects of the present invention are achieved byproviding an optical amplifier including (a) an optical amplifyingmedium to Raman amplify a wavelength division multiplex (WDM) lightincluding signal lights wavelength division multiplexed together; (b)pump light sources generating pump lights of different wavelengths; (c)a first optical multiplexer multiplexing the pump lights together; (d) asecond optical multiplexer multiplexing the WDM light with themultiplexed pump lights; (e) a decoupler decoupling a portion of theamplified WDM light; (f) a detector dividing the decoupled portion intowavelength bands and detecting a power in each wavelength band; and (g)a pump light controller controlling power of each pump light based on awavelength characteristic of gain generated in the optical amplifyingmedium for each wavelength bands, in accordance with the powers detectedby the detector.

[0023] Further, objects of the present invention are achieved byproviding an optical amplifier including (a) an optical amplifyingmedium to Raman amplify a wavelength division multiplex (WDM) lightincluding signal lights wavelength division multiplexed together; (b)pump light sources generating pump lights of different wavelengths; (c)a first optical multiplexer multiplexing the pump lights together; (d) asecond optical multiplexer multiplexing the WDM light with themultiplexed pump lights; (e) an input detector dividing the WDM lightbefore being amplified in the optical amplifying medium into wavelengthbands, and detecting the power in each wavelength band; (f) an outputdetector dividing the amplified WDM light into the same wavelength bandsas the input detector, and detecting the power in each wavelength band;and (g) a pump light controller controlling powers of the pump lightsbased on the powers detected by the input detector and the powersdetected by the output detector

[0024] In addition, objects of the present invention are achieved byproviding an optical amplifier for amplifying a wavelength divisionmultiplexed (WDM) light including signal lights wavelength divisionmultiplexed together, the amplifier including (a) an optical amplifyingmedium to Raman amplify the WDM light in accordance with multiplexedpump lights of different wavelengths traveling through the opticalamplifying medium, the WDM light being amplified in a wavelength banddivided into a plurality of individual wavelength bands; and (b) acontroller controlling power of each pump light based on a wavelengthcharacteristic of gain generated in the optical amplifying medium in theindividual wavelength bands.

[0025] Objects of the present invention are also achieved by providingan optical amplifier for amplifying a wavelength division multiplexed(WDM): light including signal lights wavelength division multiplexedtogether, the amplifier including (a) an optical amplifying medium toRaman amplify the WDM light in accordance with multiplexed pump lightsof different wavelengths traveling through the optical amplifyingmedium, the WDM light being amplified in a wavelength band divided intoa plurality of individual wavelength bands; and (b) a controllercontrolling output powers of the pump lights in accordance withdifferences in power of the WDM light before being amplified by theoptical amplifying medium and after being amplified by the opticalamplifying medium in each individual wavelength band.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] These and other objects and advantages of the invention willbecome apparent and more readily appreciated from the followingdescription of the preferred embodiments, taken in conjunction with theaccompanying drawings of which:

[0027]FIG. 1 is a diagram illustrating the relationship between a pumplight and gain wavelength during Raman amplification, according to anembodiment of the present invention.

[0028]FIG. 2 is a diagram illustrating enlargement of bandwidth of aRaman amplifier by multiplexing different wavelengths of different pumplight sources, according to an embodiment of the present invention.

[0029]FIG. 3 is a diagram illustrating a Raman amplifier, according toan embodiment of the present invention.

[0030] FIGS. 4(A), 4(B) and 4(C) are diagrams illustrating wavelengthcharacteristics of a single pump light source block of a Ramanamplifier, according to an embodiment of the present invention.

[0031] FIGS. 5(A), 5(B) and 5(C) are diagrams illustrating wavelengthcharacteristics of single pump light source block of a Raman amplifier,according to an embodiment of the present invention.

[0032] FIGS. 6(A) and 6(B) are diagrams illustrating control to obtain aconstant wavelength characteristic, according to an embodiment of thepresent invention.

[0033]FIG. 7 is a flowchart illustrating the operation of a pump lightcontroller in FIG. 3, according to an embodiment of the presentinvention.

[0034]FIG. 8 is a diagram illustrating a Raman amplifier, according toan embodiment of the present invention FIG.

[0035]FIG. 9 is a diagram illustrating a wavelength characteristic whena desired number of monitor blocks are used in a Raman amplifier,according to an embodiment of the present invention.

[0036]FIG. 10 is a diagram illustrating a practical structure of a pumplight source block and a wavelength multiplexing coupler in the Ramanamplifiers of FIGS. 3 and 8, according to an embodiment of the presentinvention.

[0037]FIG. 11 is a diagram illustrating a portion of a Raman amplifier,according to an embodiment of the present invention.

[0038]FIG. 12 is a diagram illustrating a Raman amplifier, according toan embodiment of the present invention.

[0039]FIG. 13 is a flowchart illustrating the operation of a pump lightcontroller in FIG. 12, according to an embodiment of the presentinvention.

[0040]FIG. 14 is a diagram illustrating a Raman amplifier, according toan embodiment of the present invention.

[0041]FIG. 15 is a diagram illustrating a Raman amplifier, according toan additional embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0042] A Raman amplifier is used to compensate for output tilt of anEDFA.

[0043] In addition, attention is also paid to a Raman amplifier becausethe pump light is introduced into the transmission fiber. In thismanner, the transmission fiber is used to compensate for deteriorationof output using the transmission fiber as the Raman amplifying medium,to thereby provide transmission loss wavelength compensation of thetransmission line due to the induced Raman scattering.

[0044] Raman amplifiers can mainly be considered for the following:

[0045] (1) Amplification outside of the wavelength band of EDFA.

[0046] (2) Improvement in output deviation compensation of an EDFA andimprovement in optical SNR.

[0047] (3) Induced Raman scattering compensation of the transmissionline.

[0048] In a WDM optical communication system, important characteristicsfor an optical amplifier are a wideband wavelength band, and a flatwavelength band.

[0049] It is now considered to use a plurality of pump lights ofdifferent wavelengths in view of realizing wide band transmission of aRaman amplifier. The Raman amplifier output is monitored or an outputafter insertion of an in-line amplifier after the Raman amplifier ismonitored to control outputs of a plurality of pump LDs used to attainthe band of the Raman amplifier to make small the output deviation.

[0050] When three or more pump light sources are used, the algorithms ofthe output power constant control or gain constant control andwavelength characteristic flattening control are extremely complicated.

[0051] Namely, with an increase in the number of pump wavelengths forrealizing wide band and wavelength flattening or the number of pumplight sources, more complicated control algorithms are required.Unfortunately, there are no conventionally known adequate algorithms.

[0052] Reference will now be made in detail to the present preferredembodiments of the present invention, examples of which are illustratedin the accompanying drawings, wherein like reference numerals refer tolike elements throughout.

[0053]FIG. 1 is a diagram illustrating the relationship between a pumplight and gain wavelength during Raman amplification, according to anembodiment of the present invention. Referring now to FIG. 1, pumpslights λ_(p1), λ_(p2), and λ_(p3) are pump lights for a Raman amplifier,and have associated Raman shifts of shift1, shift2 and shift 3,respectively. The center wavelength of gain and the gain bandwidth areshifted to a longer wavelength side as much as the shift of pumpwavelength when the pump wavelength is shifted to the longer wavelengthside.

[0054] Therefore, a Raman amplifier generates a gain at a respectivewavelength that is shifted in amount of Raman shift of the amplifyingmedium from the pump light wavelength. The Raman shift amount and Ramanbandwidth are intrinsically given to a substance (amplifying medium).Thus, Raman amplification is an optical amplification technique in whichgain can be obtained at any desired wavelength if a pump light sourcehaving a desired wavelength can be provided.

[0055]FIG. 2 is a diagram illustrating enlargement of bandwidth of aRaman amplifier by multiplexing different wavelengths of different pumplight sources, according to an embodiment of the present invention.Referring now to FIG. 2, a plurality of pump light sources provide pumpslights with wavelengths λ_(p1), λ_(p2), and λ_(p3), which together formpump light 100 applied to an amplifying medium. Wavelengths λ_(p1),λ_(p2), and λ_(p3) are slightly different from each other. In thismanner, gain 102 providing wideband optical amplification can berealized.

[0056]FIG. 3 is a diagram illustrating a Raman amplifier, according toan embodiment of the present invention. Referring now to FIG. 3, theRaman amplifier includes an input port 0, a Raman amplifying medium 1, amultiplexing coupler 2, a demultiplexing coupler 3, a multiplexingcoupler 4, a wavelength branching coupler 5, pump light source blocks6-1, 6-2 and 6-3, light receiving elements 7-1, 7-2 and 7-3 and a pumplight controller 8.

[0057] A wavelength division multiplexed (WDM) light 104 including aplurality of signal lights multiplexed together is incident to backpumped Raman amplifying medium 1 from the input port 0.

[0058] Multiplexing coupler 4 is a wavelength multiplexing couplermultiplexing the pump lights of average wavelength of λ_(p1), λ_(p2),and λ_(p3) of different center wavelengths from pump light source blocks6-1, 6-2 and 6-3, respectively.

[0059] Multiplexing coupler 2 is a wavelength multiplexing couplermultiplexing, in Raman amplifying medium 1, the multiplexed pump lightsfrom multiplexing coupler 4 with signal lights traveling through Ramanamplifying medium 1.

[0060] Demultiplexing coupler 3 is a light splitter demultiplexing thewavelength-multiplexed light amplified in Raman amplifying medium 1 witha ratio of, for example, 10:1.

[0061] Wavelength demultiplexing coupler 5 is a wavelength banddemultiplexing coupler demultiplexing the Raman gain wavelength bandgenerated with the pump light from pump light source blocks 6-1, 6-2 and6-3 into monitor blocks 1, 2 and 3 (not illustrated in FIG. 3). Eachmonitor block 1, 2 and 3 has a corresponding wavelength band. Lightreceiving elements 7-1, 7-2 and 7-3 receive the wavelength bands,respectively, corresponding to monitor blocks 1, 2 and 3, respectively,and perform optical/electric conversion.

[0062] Pump light controller 8 controls the output powers of averagewavelengths λ_(p1), λ_(p2), and λ_(p3) of pump light source blocks 6-1,6-2 and 6-3 in accordance with the output of the signal light receivingelements 7-1, 7-2 and 7-3.

[0063] Control performed by pump light controller 8 will be explainedbelow.

[0064] The average pump wavelength of pump light source block 6-1 isdefined as λ_(p1), and the output power of the pump light source block6-1 is defined as P_(p1). The average pump wavelength of the pump lightsource block 6-2 is defined as λ_(p2), and the output power of pumplight source block 6-2 is defined as P_(p2). The average pump wavelengthof pump light source block 6-3 is defined as λ_(p3), and the outputpower of pump light source block 6-3 is defined as P_(p3),

[0065] The average output power of the average wavelength λ_(s1) of thewavelength band of the monitor block 1 received with the light receivingelement 7-1 is defined as P_(s1). The average output power of theaverage wavelength λ_(s2) of the wavelength band of the monitor block 2received with the light receiving element 7-2 is defined as P_(s2). Theaverage output power of the average wavelength λ_(s3) of the wavelengthband of the monitor block 3 received with the light receiving element7-3 is defined as P _(s3).

[0066] FIGS. 4(A), 4(B) and 4(C) are diagrams illustrating wavelengthcharacteristics of a single pump light source block of a Ramanamplifier, according to an embodiment of the present invention.

[0067] More specifically, FIG. 4(A) is a diagram illustrating awavelength division multiplexed light output from the amplifier whenonly pump light source block 6-1 is operated in the average pumpwavelength of λ_(p1) and average pump output power of P_(p1). Referringnow to FIG. 4(A), a fine solid line 110 indicates the output spectrumwhile a thick solid line 112 indicates the average output power of eachwavelength band monitor block by driving only P_(p1).

[0068]FIG. 4(B) is a diagram illustrating a wavelength divisionmultiplex light output from the amplifier when only pump light sourceblock 6-2 is operated in the average pump wavelength λ_(p2) and averagepump output power of P_(p2). A fine solid line 114 indicates the outputspectrum while a thick solid line 116 indicates the average output powerof each wavelength band monitor block by driving only P_(p2).

[0069]FIG. 4(C) is a diagram illustrating a wavelength divisionmultiplex light output from the amplifier when only pump light sourceblock 6-3 is operated in the average pump wavelength λ_(p3) and averagepump output power of P_(p3). A fine solid line 118 indicates the outputspectrum while a thick solid line 120 indicates the average output powerof each wavelength band monitor block by driving only P_(p3).

[0070] As can be seen from FIGS. 4(A), 4(B) and 4(C), pump light sourceblock 6-1 provides a maximum contribution to the signal light output ofmonitor block 1. Pump light source block 6-2 provides a maximumcontribution to the signal light output of monitor block 2. Pump lightsource block 6-3 provides a maximum contribution to the signal lightoutput of monitor block 3.

[0071] Simultaneously, pump light source block 6-1 also makes somecontribution to the signal light output of monitor block 2 and thesignal light output of monitor block 3. Pump light source block 6-2makes some contribution to the signal light output of monitor block 1and the signal light output of monitor block 2. Pump light source block6-3 makes some contribution to the signal light output of monitor block1 and signal light output of monitor block 2.

[0072] Therefore, pump lights of a plurality of wavelengths can used toform a wideband optical amplifier. At least one of the pump lights canbe controlled, and will influence the other wavelength band monitorblocks.

[0073] In order to obtain a predetermined amplified signal power, a gaincoefficient is multiplied by the power of a pump light source.Therefore, when the average power variation of the pump light outputs ofthe pump light source blocks 6-1 to 6-3 is defined as αP_(p), thevariation of average output power of the band in which the gain isgenerated with the pump lights from the light receiving elements 7-1 to7-3 is defined as ΔP_(s) and the average gain coefficient is defined asA, the following Formula 1 can be determined.

[0074] Formula 1

ΔP _(s) =A·ΔP _(p)

[0075] To eliminate output power wavelength characteristic deviation ofeach wavelength block, ΔP_(P) can be adjusted to make identical thepower levels of the wavelength-multiplex signal lights of wavelengthbands demultiplexed into three bands with the wavelength demultiplexingcoupler 5. ΔP_(P) can be adjusted, for example, by varying an opticaloutput power of the pump light source, by varying the pump wavelength toshift the center of gravity wavelength and also by varying the pumplight wavelength width. Here, an example of adjustment for varying anoptical output power will be explained.

[0076] As illustrated in FIGS. 4(A), 4(B) and 4(C), since the gainwavelength band generated by one pump light source block is wide and thegain is generated over each monitor block, when one pump light sourceblock is varied, Formula 1 must be calculated, considering the influenceon the wavelength of the other monitor blocks.

[0077] In other words, regarding the power of each monitor block, anoutput power of each pump light source block should be controlled basedon the wavelength characteristic of the gain generated in the opticalamplifying medium of each pump light source block.

[0078] Here, the average gain coefficient of the average output powervariation ΔP_(P) of the pump wavelength λ_(p1) of the pump light sourceblock 6-1 affecting on the average output power variation ΔP_(s1) of themonitor block 1 is defined as A₁₁. The average gain coefficient of theaverage output power variation ΔP_(p1) of the pump wavelength λ_(p1) ofthe pump light source block 6-1 affecting on the average output powervariation P_(p2) of the monitor block 2 is defined as A₂₁. The averagegain coefficient of the average output power variation ΔP_(p1) of thepump wavelength λ_(p1) of the pump light source block 6-1 affecting onthe average output power variation ΔP_(s3) of the monitor block 3 isdefined as A₃₁.

[0079] The average gain coefficient of the average output powervariation ΔP_(p2) of the pump wavelength λ_(p2) of the pump light sourceblock 6-2 affecting on the average output power variation ΔP_(s1) of theblock 1 of the monitor block is defined as A₁₂. The average gaincoefficient of the average output power variation ΔP_(p2) of the pumpwavelength λ_(p2) of the pump light source block 6-2 affecting on theaverage output power variation ΔP_(s2) of the monitor block 2 is definedas A₂₂. The average gain coefficient of the average output powervariation ΔP_(p2) of the pump wavelength λ_(p2) of the pump light sourceblock 6-2 affecting on the average output power variation ΔP_(s3) of themonitor block 3 is defined as A₃₂.

[0080] The average gain coefficient of the average output powervariation ΔP_(p3) of the pump wavelength λ_(p3) of the pump light sourceblock 6-3 affecting on the average output power variation ΔP_(s1) of themonitor block 1 is defined as A₁₃. The average gain coefficient of theaverage output power variation ΔP_(p3) of the pump wavelength λ_(p3) ofthe pump light source block 6-3 affecting on the average output powervariation ΔP_(s2) of the monitor block 2 is defined as A₂₃. The averagegain coefficient of the average output power variation ΔP_(p3) of thepump wavelength λ_(p3) of the pump light source block 6-3 affecting onthe average output power variation AP, of the monitor block 3 is definedas A₃₃.

[0081] FIGS. 5(A), 5(B) and 5(C) are diagrams illustrating wavelengthcharacteristics of a single pump light source block of a Ramanamplifier, according to an embodiment of the present invention.

[0082] More specifically, FIG. 5(A) illustrates the average output powerdifference of the monitor block 1, the monitor block 2 and the monitorblock 3 for the pump light output power difference when only the pumplight source block 6-1 is operated. Respective gradients correspond toA₁₁, A₂₁, A₃₁.

[0083]FIG. 5(B) illustrates the average output power difference of themonitor block 1, the monitor block 2 and the monitor block 3 for thepump light output power difference when only the pump light source block6-2 is operated. Respective gradients correspond to A₁₂, A₂₂, A₃₂.

[0084]FIG. 5(C) illustrates the average output power difference of themonitor block 1, the monitor block 2 and the monitor block 3 for thepump light output power difference when only the pump light source block6-3 is operated. Respective gradients correspond to A₁₃, A₂₃, A₃₃.

[0085] Here, the average gain coefficient matrix [A] including theseelements can be obtained. $\begin{matrix}{\begin{bmatrix}{\Delta \quad P_{S1}} \\{\Delta \quad P_{S2}} \\{\Delta \quad P_{S3}}\end{bmatrix} = {\begin{bmatrix}A_{11} & A_{12} & A_{13} \\A_{21} & A_{22} & A_{23} \\A_{31} & A_{32} & A_{33}\end{bmatrix}\begin{bmatrix}{\Delta \quad P_{P1}} \\{\Delta \quad P_{P2}} \\{\Delta \quad P_{P3}}\end{bmatrix}}} & {{Formula}\quad 2}\end{matrix}$

[0086] FIGS. 6(A) and 6(B) are diagrams illustrating control to obtain aconstant wavelength characteristic, according to an embodiment of thepresent invention.

[0087] Referring now to FIG. 6(A), the average output of the monitorblock 1, the monitor block 2 and the monitor block 3 when the wavelengthcharacteristic of the signal light output has a large signal lightspectrum is indicated with a thick solid line 122 and the average outputP_(f) of the total wavelength band is indicated with a broken line 124.

[0088] Reduction of the wavelength characteristic deviation of thewavelength multiplex light output indicates that the average outputsP_(s1), P_(s2), and P_(s3) of monitor blocks 1, 2 and 3, respectively,are matched, as illustrated in FIG. 6(B), with the targetRaman-amplified wavelength multiplex light output P_(f) (average outputof total wavelength band).

[0089] Formula 3

ΔP _(s1) =|P _(f) −P _(s1)|

ΔP _(s2) =|P _(f) −P _(s2)|

ΔP _(s3) =|P _(f) −P _(s3)|

[0090] Formula 4

ΔP _(s1) ≈ΔP _(s2) ≈ΔP _(s3)

[0091] Output difference (tilt) can be suppressed small in the totalwavelength band where the Raman gain is generated in the Ramanamplifying medium 1 by calculating the compensation amount of the pumplight outputs P_(p1), P_(p2) an P_(p3) of the pump light source blocks6-1, 6-2 and 6-3, respectively, to satisfy the above formula.$\begin{matrix}{\begin{bmatrix}{\Delta \quad P_{P1}} \\{\Delta \quad P_{P2}} \\{\Delta \quad P_{P3}}\end{bmatrix} = {\begin{bmatrix}A_{11} & A_{12} & A_{13} \\A_{21} & A_{22} & A_{23} \\A_{31} & A_{32} & A_{33}\end{bmatrix}^{- 1}\begin{bmatrix}{\Delta \quad P_{S1}} \\{\Delta \quad P_{S2}} \\{\Delta \quad P_{S3}}\end{bmatrix}}} & {{Formula}\quad 5}\end{matrix}$

[0092] Namely, it is enough for the pump light controller 8 of FIG. 3 tocontrol the pump light power output from each pump light source block6-1, 6-2 and 6-3 by (a) monitoring the output power by dividing thewavelength-multiplex light where a plurality of signal lights arewavelength-multiplexed into the monitor blocks of the predeterminedwavelength band, (b) executing the average value process obtained bydividing total output of the monitor block of each wavelength band withthe number of channels, and (c) calculating, with the Formula 5, theaverage output power difference (tilt) of the pump light for weightingthe influence on the wavelength of each monitor block of the pumpwavelength of each pump light source block required to reduce the outputpower difference in the total wavelength band.

[0093] Moreover, the feedback control might typically be performed, forexample, up to about ten (10) times until the predetermined wavelengthcharacteristic deviation is obtained.

[0094] With these control processes, the average power of the Raman gainwavelength band generated with the pump light can be set to the constantpower P_(f).

[0095]FIG. 7 is a flowchart illustrating a process performed by pumplight controller 8 in FIG. 3, according to an embodiment of the presentinvention. The processes in FIG. 7 can be performed, for example, such aprocessor, such as a CPU.

[0096] Referring now to FIG. 7, in operation 1, the control process isstarted.

[0097] From operation 1, the process moves to operation 2, where theaverage output powers P_(s1), P_(s2) and P_(s3) in the monitor blocks 1,2 and 3, respectively, are obtained from the outputs of the lightreceiving elements 7-1, 7-2 and 7-3, respectively.

[0098] From operation 2, the process moves to operation 3, whereΔP_(s1), ΔP_(s2) and ΔP_(s3) are obtained by comparing the averagewavelength output powers P_(s1), P_(s2) and P_(s3) in the monitor block1, 2 and 3, respectively, with the target wavelength multiplex outputvalue P_(f).

[0099] From operation 3, the process moves to operation 4, where it isdetermined whether the difference between ΔP_(s1) to ΔP_(s3) and P_(f)is within an allowable range. If the difference is within the allowablerange, the process moves to operation 7 where the process stops. If thedifference is not within the allowable range, the process moves tooperation 5, where control amounts ΔP_(p1), ΔP_(p2) and ΔP_(p3) of thepower levels P_(p1), P_(p2) and Pp₃ of the pump light source blocks 6-1,6-2 and 6-3 are obtained, from ΔP_(s1), ΔP_(s2), ΔP_(s3), using theinverse matrix of the average gain coefficients A₁₁ to A₃₃ which areaffected on each monitor block by each pump light.

[0100] From operation 5, the process moves to operation 6, where theoutput powers P_(p1), P_(p2) and P_(p3) of the pump light source blocks6-1, 6-2, 6-3, respectively, are controlled by adding the controlamounts ΔP_(p1), ΔP_(p2), ΔP_(p3) to the current P_(p1), P_(p2), P_(p3),respectively.

[0101] From operation 6, the process moves to operation 7, where controlprocess is completed.

[0102] In FIG. 3, as an example, a total pump light source block isprovided as the three pump light source blocks 6-1, 6-2 and 6-3, and thetotal monitor block of the wavelength band that generates the gainthrough the pump light from the pump light source block is divided intothree monitor blocks. However, the present invention is not limited to atotal pump light source block provided “three” pump light source blocks,or a total monitor block as divided into “three” monitor blocks.Instead, the number of pump light source blocks of the total pump lightsource block and the number of monitor blocks of the total monitor blockcan be set to any practical number, which would typically be a matter ofdesign choice.

[0103] For example, FIG. 8 is a diagram illustrating a Raman amplifier,according to an additional embodiment of the present invention. In FIG.8, the number of pump light source blocks and monitor blocks can be setfreely. Thus, in FIG. 8, n pump light source blocks (6-1 to 6-n) and mmonitor blocks of the wavelength multiplex signal light are provided.

[0104]FIG. 9 is a diagram illustrating a wavelength characteristic whena desired number of monitor blocks are used in a Raman amplifier,according to an embodiment of the present invention. More specifically,FIG. 9 illustrates the wavelength band of the Raman amplification gainfor the wavelength division multiplexed light decoupled by wavelengthdemultiplex coupler 5 in FIG. 8, with the wavelength band being dividedinto m monitor blocks.

[0105] Variation ΔP_(p) of the pump light power control is expressed asan n×1 matrix. Difference ΔP_(s) between the average value of thewavelength multiplex signal light power in the monitor block and thetarget control value is expressed as the m×1 matrix. A is expressed asthe n×m matrix. $\begin{matrix}{\begin{bmatrix}{\Delta \quad P_{S_{1}}} \\{\Delta \quad P_{S_{2}}} \\\vdots \\{\Delta \quad P_{S_{n}}}\end{bmatrix} = {\begin{bmatrix}A_{11} & A_{12} & \cdots & A_{1m} \\A_{21} & A_{22} & \cdots & A_{2m} \\\vdots & \vdots & \cdots & \vdots \\A_{n1} & A_{n2} & \cdots & A_{n\quad m}\end{bmatrix}\begin{bmatrix}{\Delta \quad P_{P_{1}}} \\{\Delta \quad P_{P_{2}}} \\\vdots \\{\Delta \quad P_{P_{M}}}\end{bmatrix}}} & {{Formula}\quad 6}\end{matrix}$

[0106] ΔP_(pi), in this case, is variation of the average output powerof the pump light source, while ΔP_(sj) is variation of the averageoutput power of the signal light monitor block.

[0107] Since variation is used, it is not required to convert themonitor output power to the main signal output power.

[0108] Here, it is understood that ΔP_(pi) resulting from ΔP_(sj) can beobtained by obtaining the inverse matrix [A]⁻¹ of [A]. $\begin{matrix}{\begin{bmatrix}{\Delta \quad P_{P_{1}}} \\{\Delta \quad P_{P_{2}}} \\\vdots \\{\Delta \quad P_{P_{m}}}\end{bmatrix} = {A^{- 1}\begin{bmatrix}{\Delta \quad P_{S_{1}}} \\{\Delta \quad P_{S_{2}}} \\\vdots \\{\Delta \quad P_{S_{n}}}\end{bmatrix}}} & {{Formula}\quad 7}\end{matrix}$

[0109] Therefore, reduction of deviation of the average output poweramong each block indicates flattening of the wavelength characteristicof the signal light output power.

[0110] In the embodiment of FIG. 3, the desired number of pump lightsource blocks and monitor blocks is not limited to any particular numberand can be determined in accordance with design choice. However, it ispreferable that the number of monitor blocks be less than the number ofsignal light channels multiplexed to the wavelength multiplex light, andexceeding the number of pump light source blocks.

[0111]FIG. 10 is a diagram illustrating a practical structure of a pumplight source block and a wavelength multiplexing coupler in the Ramanamplifiers of, for example, FIGS. 3 and 8, according to an embodiment ofthe present invention. Referring now to FIG. 10, the embodiment includesWDM couplers 24 and 25, deflection composite couplers 61, 62 and 63,fiber grating filters 51, 52, 53, 54, 55 and 56, and semiconductorlasers 81, 82, 83, 84, 85 and 86.

[0112] The pump light source block 6-1 includes semiconductor lasers 81and 82. The pump light source block 6-2 includes semiconductor lasers 83and 84. The pump light source block 6-3 includes semiconductor lasers 85and 86. Semiconductor lasers 81 and 82 have slightly differentwavelengths. Semiconductor lasers 83 and 84 have slightly differentwavelengths. Semiconductor lasers 85 and 86 have slightly differentwavelengths. In the example of FIG. 10, the various pairs ofsemiconductor lasers have wavelengths which are about 4 nm apart, butthe present invention is not limited to this specific wavelengthdifference.

[0113] The pump lights from the semiconductor lasers 81 and 82 are at,for example, wavelengths 1429.7 nm and 1433.7 nm, respectively, and arereflected at the fiber grating filters 51 and 52, respectively, toprovide a resonance structure to output a pump light of the particularwavelength. PBS coupler 61 multiplexes these pump lights, to provide apump light provided by pump light source block 6-1.

[0114] The pump lights from the semiconductor lasers 83 and 84 are at,for example, wavelengths 1454.0 nm and 1458.0 nm, respectively, and arereflected at the fiber grating filters 53 and 54, respectively, toprovide a resonance structure to output a pump light of the particularwavelength. PBS coupler 62 multiplexes these pump lights, to provide apump light provided by pump light source block 6-2.

[0115] The pump lights from the semiconductor lasers 85 and 86 at, forexample, wavelengths 1484.5 nm and 1488.5 nm, respectively, and arereflected at the fiber grating filters 55 and 56, respectively, toprovide a resonance structure to output a pump light of the particularwavelength. PBS coupler 63 multiplexes these pump lights, to provide apump light provided by pump light source block 6-3.

[0116] The polarization coupling by PBS couplers 61, 62 and 63 isperformed, for example, to eliminate dependence on change of the Ramanamplification.

[0117] The multiplex coupler 4 includes the WDM couplers 24 and 25. TheWDM coupler 25 operates, for example, by reflecting the wavelength lightfrom the pump light source block 6-2 and transferring the wavelengthfrom the pump light source block 6-3. The WDM coupler 24 operates, forexample, by reflecting the wavelength light from the pump light sourceblock 6-1 and transferring the wavelength from the pump light sourceblock 6-3.

[0118] In FIG. 10, in each pump light source block 6-1, 6-2 and 6-3, thevarious semiconductor laser-fiber grating pairs output light which isslightly different in wavelength from each other. However, the presentinvention is not limited to this, and equal wavelength can be output.Moreover, the light of each pump light source block is not required tobe formed with a plurality of semiconductor lasers. For example, a pumplight of a pump light source block can be formed by a single lightsource which does not depend on polarization.

[0119] In FIG. 3, the target wavelength multiplex light output value isdefined as P_(f) and the average powers of all wavelength bands arecontrolled to become equal to P_(f). Therefore, it is possible toperform control to obtain constant output in all wavelength bands.

[0120] As a modified example of this constant output control, P_(f) isdefined as P_(f1), P_(f2), P_(f3) for each wavelength band, or monitorblock, of the total monitor block and these values are compared toconduct individual constant output control in the individual monitorblocks.

[0121] In this case, P_(f1), P_(f2), P_(f3) correspond to monitor blocks1, 2 and 3, respectively, in place of P_(f) in operation 4 of theflowchart of FIG. 7.

[0122] The pump light controller 8 may also be controlled by subtractingthe corresponding P_(s1), P_(s2), P_(s3) from the values P_(f1), P_(f2),P_(f3).

[0123]FIG. 11 is a diagram illustrating a portion of a Raman amplifier,according to an embodiment of the present invention. Referring now toFIG. 11, weighting can be performed freely in monitor blocks 1, 2 and 3to conduct constant output control individually in monitor blocks 1, 2and 3, by providing, in place of changing P_(f), variable or fixedattenuators 71, 72 and 73 in the preceding stage of the light receivingelements 7-1, 7-2 and 7-3 of FIG. 3.

[0124] Moreover, the embodiment in FIG. 3 can freely use, as the Ramanamplifying medium, for example, dispersion compensation fiber (DCF)resulting in small effective sectional area and large non-linearity,dispersion shifted fiber (DSF) and non-zero dispersion shifted fiber(NZDSF), as well as the ordinary 1.3 zero-micron fiber.

[0125] When fibers having large non-linearity are used, the length ofthe fiber that operates as the Raman amplifying medium to obtain thenecessary gain can be shortened. Therefore, centralized amplificationcan be realized.

[0126] In the embodiment of FIG. 3, the wavelength demultiplex couplers3 and 5, and light receiving elements 7-1, 7-2 and 7-3, are used toprovide a monitor block. Instead, however, a spectrum analyzer can beused.

[0127]FIG. 12 is a diagram illustrating a Raman amplifier, according toan additional embodiment of the present invention. In FIG. 12, abranching coupler 9, a wavelength demultiplexing coupler 10 and lightreceiving elements 11-1, 11-2 and 11-3 are also used to provide amonitor block, in addition to elements of FIG. 3.

[0128] In FIG. 12, a plurality of wavelength-multiplexed signals areprovided the input port 0 of the Raman amplifier. The branching coupler9 is a light splitter provided at the input port 0 to branch thewavelength-multiplexed signals by, for example, a 10:1 ratio.

[0129] The wavelength demultiplexing coupler 10 is a wavelength bandbranching coupler for dividing the Raman gain wavelength band generatedfrom the pump light transmitted from the pump light source blocks 6-1,6-2 and 6-3 into the three wavelength bands (monitor blocks), in asimilar manner as the wavelength demultiplexing coupler 5. Namely,wavelength demultiplexing coupler 10 is a wavelength demultiplexingfilter for demultiplex the Raman gain wavelength band into monitorblocks 1, 2 and 3 of the wavelength band.

[0130] The light receiving elements 11-1, 11-2 and 11-3 convert theoptical power of the monitor blocks 1, 2 and 3, respectively.

[0131] Regarding monitor blocks 1, 2 and 3 isolated by the wavelengthdemultipexing coupler 10, the average output power of the averagewavelength λ_(s1) of the monitor block 1 is defined as P_(in) _(—)_(s1), the average output power of the average wavelength λ_(s2) of themonitor block 2 is defined P_(in) _(—) _(s2), and the average outputpower of the average wavelength λ_(s3) of the monitor block 3 is definedas P_(in) _(—) _(s3).

[0132] The main signal light is incident to the back pumped Ramanamplifying medium 1.

[0133] The pump light source blocks 6-1, 6-2 and 6-3 may be constructed,for example, as illustrated in FIG. 10 or may be realized in variousembodiments like that for the embodiment in FIG. 3.

[0134] The signal amplified with the amplifying medium 1 is branchedwith branching coupler 3 by, for example, a 10:1 ratio, and divided intothe three wavelength band blocks like that of the wavelengthdemultiplexing coupler 10.

[0135] The wavelength band of the wavelength demultiplexing coupler 5respectively corresponds to the average wavelengths λ_(s1), λ_(s2),λ_(s3) of the monitor block of the wavelength branching coupler 10. Thewavelength multiplex output power is photo-electrically converted in thelight receiving elements 7-1, 7-2 and 7-3.

[0136] As with FIG. 3, the average output power of the averagewavelength λs1 of the monitor block 1 of the wavelength demultipexingcoupler 5 is defined as P_(s1), the average output power of the averagewavelength λ_(s2) of the monitor block 2 is defined as P_(s2), and theaverage output power of the average wavelength λ_(s3) of the monitorblock 3 is defined as P_(s3).

[0137] The pump light controller 8 controls the gain to a predeterminedvalue with the monitor input from the light receiving elements 7-1, 7-2,7-3, 11-1, 11-2 and 11-3.

[0138] Practical operations of the pump light controller 8 in FIG. 12are explained below.

[0139] The average gains G₁, G₂, G₃ of monitor blocks 1, 2 and 3,respectively, can be obtained by subtracting P_(in) _(—) _(s1), P_(in)_(—) _(s2), P_(in) _(—) _(s3), obtained with the light receivingelements 11-1, 11-2 and 11-3 through isolation with the wavelengthdemultiplexing coupler 10 in the input port side from P_(s1), P_(s2),P_(s3) obtained with the light receiving elements 7-1, 7-2 and 7-3,respectively

[0140] Formula 8

G ₁ =P _(s1) −P _(in) _(—) _(s1)

G ₂ =P _(s2) =P _(in) _(—) _(s2)

G ₃ =P _(s3) −P _(in) _(—) _(s1)

[0141] The pump light average output power of each monitor block and thewavelength light average gain of each monitor block may be coupled withthe average gain coefficient of each monitor block and when the pumplight average output power variation amount is ΔP_(p), the signal lightaverage output power variation amount is ΔG, and the average gaincoefficient is A.

[0142] Formula 9

ΔG=A·ΔP _(p)

[0143] [A] used in the embodiment for FIG. 3 indicates gradient of thesignal light average output power of the pump light average outputpower. Therefore, the following relationship can also be established forthe gain A defined here. $\begin{matrix}{\begin{bmatrix}{\Delta \quad G_{1}} \\{\Delta \quad G_{2}} \\{\Delta \quad G_{3}}\end{bmatrix} = {\begin{bmatrix}A_{11} & A_{12} & A_{13} \\A_{21} & A_{22} & A_{23} \\A_{31} & A_{32} & A_{33}\end{bmatrix}\begin{bmatrix}{\Delta \quad P_{p1}} \\{\Delta \quad P_{p2}} \\{\Delta \quad P_{p3}}\end{bmatrix}}} & {{Formula}\quad 10}\end{matrix}$

[0144] Here, the target gain level is defined as average gain G_(f) ofthe total wavelength band, the average gain of each monitor block isdefined as G₁, G₂, G₃, the difference of G_(f) and G₁ is defined as ΔG₁,the difference of G_(f) and G₂ as ΔG₂ and the difference of G_(f) and G₃as ΔG₃.

[0145] Formula 11

ΔG ₁ =|G _(f) −G ₁|

ΔG ₂ =|G _(f) −G ₂|

ΔG ₃ =|G _(f) −G ₃|

[0146] In order to make small the gain wavelength deviation (tilt) inthe total wavelength band, the average gain among monitor blocks is setequally to match with the average gain G_(f) of the total wavelengthband.

[0147] Here, all wavelengths can be controlled to the constant gain bysetting G_(f) to the predetermined value for obtaining the constantgain.

[0148] Formula 12

[0149] ΔG ₁ ≈ΔG ₂ ≈ΔG ₃

[0150] Therefore, it is possible to calculate ΔP_(p1), ΔP_(p2), ΔP_(p3)from the Formula 13 using the Formula 11. $\begin{matrix}{\begin{bmatrix}{\Delta \quad P_{p1}} \\{\Delta \quad P_{p2}} \\{\Delta \quad P_{p3}}\end{bmatrix} = {\begin{bmatrix}A_{11} & A_{12} & A_{13} \\A_{21} & A_{22} & A_{23} \\A_{31} & A_{32} & A_{33}\end{bmatrix}^{- 1}\begin{bmatrix}{\Delta \quad G_{1}} \\{\Delta \quad G_{2}} \\{\Delta \quad G_{3}}\end{bmatrix}}} & {{Formula}\quad 13}\end{matrix}$

[0151] Namely, the pump light controller 8 obtains total output of themonitor block of the wavelength multiplex light, executes the process toobtain the average value by dividing total output of the monitor blockwith the number of channels and controls the pump light source blocks ofthe monitor block by calculating the necessary average output differenceof pump light considering the influence of the gain by each pump lightsource block on the wavelength of each monitor block in view of makingsmall the gain difference in the total wavelength band.

[0152] The feedback controls are repeated, for example, up to ten (10)times until the wavelength characteristic deviation (tilt) of the gainof each monitor block of the Raman optical amplifier is eliminated.

[0153]FIG. 13 is a flowchart illustrating the operation of the pumplight controller 8 in FIG. 12, according to an embodiment of the presentinvention. Referring now to FIG. 13, in operation 1, the control isstarted.

[0154] From operation 1, the process moves to operation 2, where thegains G₁, G₂ and G₃ of the monitor block are obtained, respectively, bysubtracting the powers P_(in) _(—) _(s1), P_(in) _(—) _(s2), P_(in) _(—)_(s3), of the monitor blocks of the wavelength demultiplexing coupler 5provided in the input side from the powers P_(s1), P_(s2) and P_(s3),respectively, of the monitor blocks of the wavelength demultiplexingcoupler 5 provided in the output side of the optical amplifying medium1.

[0155] From operation 2, the process moves to operation 3, where thetarget gain G_(f) is compared with the gains G₁, G₂ and G₃ in themonitor blocks to obtain the differences.

[0156] From operation 3, the process moves to operation. 4, where thedifference between ΔG₁ ΔG₂ and G_(f) is determined. When difference iswithin an allowable range in operation 4, the process moves to operation7, where the process stops. When difference is not within the allowablerange in operation 4, the process moves to operation 5.

[0157] In operation 5, control amounts ΔP_(p1), ΔP_(p2) and ΔP_(p3) ofthe power levels Pp1, Pp2 and Pp3, respectively, of the pump lightsource blocks λ_(p1), λ_(p2) and λ_(p3), respectively, are obtained fromΔG₁, ΔG₂ and ΔG₃ using the average gain coefficients A₁₁ to A₃₃ whichaffects on each monitor block with each pump light.

[0158] From operation 5, the process moves to operation 6, where theoutput powers P_(p1), P_(p2) and P_(p3) of the pump light source blocks6-1, 6-2 and 6-3, respectively, are controlled by adding the controlamounts ΔP_(p1), ΔP_(p2) and ΔP_(p3) to the current P_(p1), P_(p2) andP_(p3), respectively.

[0159] With the flow explained above, the pump light controller 8controls the individual pump light source blocks. In the embodiment ofFIG. 12, like the embodiment of FIG. 3, the number of pump light sourceblocks and monitor blocks may be set freely.

[0160] Namely, when the number of pump light source blocks is set to n,while the number of monitor blocks is set to m, the Formula 10, Formula11, Formula 12 and Formula 13 may be updated as follows.

[0161] Formula 14

G₁ =P _(s1) −P _(in) _(—) _(s1)

G₂ =P _(s2) −P _(in) _(—) _(s2)

G_(m) =P _(sm) −P _(in) _(—) _(sm) $\begin{matrix}{\begin{bmatrix}{\Delta \quad G_{1}} \\{\Delta \quad G_{2}} \\\vdots \\{\Delta \quad G_{m}}\end{bmatrix} = {\begin{bmatrix}A_{11} & A_{12} & \cdots & A_{1n} \\A_{21} & A_{22} & \cdots & A_{2n} \\\vdots & \vdots & \cdots & \vdots \\A_{m1} & A_{m2} & \cdots & A_{mn}\end{bmatrix}\begin{bmatrix}{\Delta \quad P_{P_{1}}} \\{\Delta \quad P_{P_{2}}} \\\vdots \\{\Delta \quad P_{P_{n}}}\end{bmatrix}}} & {{Formula}\quad 15}\end{matrix}$

[0162] Formula 16

ΔG ₁ =|G _(f) −G ₁|

ΔG ₂ =|G _(f) −G ₂|

ΔG _(m) =|G _(f) −G _(m)|

[0163] Formula 17

ΔG ₁ ≈ΔG ₂ ≈ΔG _(m) $\begin{matrix}{\begin{bmatrix}{\Delta \quad P_{P_{1}}} \\{\Delta \quad P_{P_{2}}} \\\vdots \\{\Delta \quad P_{pn}}\end{bmatrix} = {A^{- 1}\begin{bmatrix}{\Delta \quad G_{1}} \\{\Delta \quad G_{2}} \\\vdots \\{\Delta \quad G_{m}}\end{bmatrix}}} & {{Formula}\quad 18}\end{matrix}$

[0164] Thus, the pump light controller 8 could be designed in accordancewith the above formula.

[0165] In the embodiment of FIG. 12, the number of pump light sourceblocks and monitor blocks can be set freely as in the case of theembodiment of FIG. 3, but it is preferable that the number of monitorblocks is set less than the number of signal light channels multiplexedin the wavelength multiplex signal, and exceeding the number of pumplight source blocks.

[0166] Moreover, as with the embodiment of FIG. 3, the embodiment ofFIG. 12 can freely use, as the Raman amplifying medium, a dispersioncompensation fiber. (DCF) resulting in small effective sectional areaand large non-linearity, a dispersion shift fiber (DSF) and a non-zerodispersion shift fiber (NZDSF) as well as the ordinary 1.3 zero-micronfiber.

[0167] When an optical fiber operating as the Raman amplifying medium 1has a large non-linearity, the fiber can be relatively short in length,while providing centralized amplification.

[0168] Moreover, when an optical fiber operating as the Raman amplifyingmedium 1 has a small effective cross-sectional area and intensivenon-linearity, the Raman amplifying medium 1 can be structured in shortlength. However, when an ordinary 1.3 μm zero-discrete fiber is used, alength of about 40 km or longer will probably be required depending onthe pump power.

[0169]FIG. 14 is a diagram illustrating a Raman amplifier, according toa further embodiment of the present invention. More specifically, FIG.14 illustrates an example where input of the wavelength multiplex lightof the Raman amplifier of FIG. 12 is notified using the actualtransmission line.

[0170] Referring now to FIG. 14, a monitor controller (OSC) 12 detectsthe power of each monitor block and transmits information of the resultto the Raman amplifying medium 1 as the transmission line via amultiplexing coupler 13 in the wavelength of λ_(osc). The signal ofwavelength λ_(osc) is demultiplexed with the wavelength demultiplexingcoupler 5 and detected with a monitor controller (OSC) 14 and is thensupplied to the pump optical controller 8.

[0171] In FIG. 14, the wavelength λ_(osc) is demultiplexed with thewavelength demultiplexing coupler 5, but it is possible to additionallyprovide a branching coupler to the transmission line and to branch themonitor control signal and input this signal to the monitor controller14.

[0172] In FIG. 13, the gain can be kept constant even when the Ramanamplifying medium 1 is used in the transmission line through theembodiment explained above in the gain wavelength band of the amplifyingmedium 1 with the pump light from the pump light source block using thesame value of G_(f) for all monitor blocks. The gain weighted for eachwavelength band of each monitor block can be controlled to the constantvalue by setting the other gain G_(f) for each monitor block.

[0173] In addition, as with the embodiment in FIG. 3, with theembodiment in FIG. 12, the weighting process can be performed constantlyto G_(f) for all wavelength blocks and it is also possible to conductthe weighting process by providing variable or fixed optical attenuators71 to 73 in the preceding stage of the light receiving elements in unitof monitor block.

[0174] In the embodiment of FIG. 12, the monitor block is formed via thewavelength demultiplexing couplers 5 and 10, and light receivingelements 7-1, 7-2, 7-3, 11-1, 11-2 and 11-3, but these may be replacedwith the spectrum analyzers.

[0175] The embodiments in FIGS. 3 and 13 can be combined with an opticalamplifier using a rare-earth doped fiber (for example, an erbium-dopedfiber).

[0176] For example, FIG. 15 is a diagram illustrating a Raman amplifier,according to an additional embodiment of the present invention.Referring now to FIG. 15, a first rare-earth doped fiber amplifier 13-1,a second rare-earth doped fiber amplifier 13-2, a wavelength banddemultiplexing coupler 5-1, branching couplers 5-2, 5-3, 5-4 and 5-5, afirst wavelength band monitor 5-6, a second wavelength band monitor 5-7,a first spectrum analyzer 5-8 and a second spectrum analyzer 5-9 areprovided.

[0177] The wavelength band demultiplexing coupler 5-1 divides thewavelength multiplex light amplified with the Raman amplifying medium 1to a first wavelength band (C-band: 1530 nm to 1557 nm) and a secondwavelength band (L-band: 1570 nm to 1610 nm) and then outputs thesedivided wavelength bands.

[0178] The first rare-earth doped fiber amplifier 13-1 is an opticalamplifier formed of an erbium-doped fiber (EDF) having the gain for thefirst wavelength band. The second rare-earth doped fiber amplifier 13-2is an optical amplifier formed of an erbium-doped fiber (EDF) having thegain for the second wavelength band.

[0179] One light branched with the wavelength band demultiplexingcoupler 5-1 is amplified by the first rare-earth doped fiber amplifierin the first wavelength band, and the other light branched withwavelength band demultiplexing coupler 5-1 is amplified by the secondrare-earth doped fiber amplifier in the second wavelength band.

[0180] The branching couplers 5-2, 5-3 are branching couplers forbranching the light of the first wavelength band in the ratio of, forexample, 10:1. The branching couplers 5-4, 5-5 are branching couplersfor branching the light of the second wavelength band in the ratio of,for example, 10:1.

[0181] The first wavelength band monitor 5-6 monitors the power of thefirst wavelength band light branched with the branching coupler 5-2. Thesecond wavelength band monitor 5-7 monitors the power of the secondwavelength band light branched with the branching coupler 5-4.

[0182] The pump light controller 8 calibrates the output powers of thefirst spectrum analyzer 5-8 and second spectrum analyzer 5-9 based onthe outputs of the first and second wavelength monitors 5-6 and 5-7.Outputs of the spectrum analyzers 5-8 and 5-9 are divided to thewavelength band blocks of, for example, 1528.773 to 1552.122 nm,1552.524 to 1563.455 nm, 1570.416 to 1581.601 nm, and 1582.018 to1607.035 nm, to obtain the average output of each monitor block in viewof controlling the pump lights 6-1, 6-2 and 6-3.

[0183] In the embodiment of FIG. 12, it is possible to use an output ofthe wavelength demultiplexing coupler 10 of FIG. 12 and FIG. 14 and usea method for detecting the signal before Raman amplification from themonitor controller 14 using the monitor control wavelength signal OSC.

[0184] According to the above embodiments of the present invention, aplurality of pump light sources are used to realize a wideband Ramanamplifier with flattening of the wavelength characteristic of output andgain. The present invention enables control of wavelength characteristicdeviation of output power and gain, control of constant output, andcontrol of constant gain using a simplified control algorithm. Invarious embodiments of the present invention, the number of wavelengthbands for monitoring an amplified light are higher than the number ofindividual blocks forming a pump light source block and lower than thenumber of signal channels.

[0185] In the various examples provided herein, specific wavelengths,frequencies and other values are provided for explanation purposes.However, the present invention is not limited to such specificwavelengths, frequencies or other values.

[0186] Although a few preferred embodiments of the present inventionhave been shown and described, it would be appreciated by those skilledin the art that changes may be made in these embodiments withoutdeparting from the principles and spirit of the invention, the scope ofwhich is defined in the claims and their equivalents.

What is claimed is:
 1. An optical amplifier for amplifying a wavelengthdivision multiplexed (WDM) light including signal lights wavelengthdivision multiplexed together, the amplifier comprising: an opticalamplifying medium through which the WDM light travels and is therebyamplified via Raman amplification in accordance with multiplexed pumplights of different wavelengths traveling through the optical amplifyingmedium; an optical device dividing the amplified WDM light into firstand second divided lights in first and second wavelength bands,respectively; a first optical amplifier amplifying the first dividedlight, the first optical amplifier having a gain band including thefirst wavelength band; a second optical amplifier amplifying the seconddivided light, the second optical amplifier having a gain band includingthe second wavelength band; and a controller controlling power of eachpump light based on a power of the first divided light amplified by thefirst optical amplifier and a power of the second divided lightamplified by the second optical amplifier.
 2. An optical amplifier asclaimed in claim 1, wherein the first wavelength band is divided into aplurality of first individual wavelength bands, the second wavelengthband is divided into a plurality of second individual wavelength bands,and the controller controls power of each pump light based on awavelength characteristic of gain generated in the first individualwavelength bands and the second individual wavelength bands.
 3. Anoptical amplifier as claimed in claim 1, wherein the first wavelengthband is C-band and the second wavelength band is L-band.
 4. An opticalamplifier as claimed in claim 1, wherein the first wavelength band isC-band, the second wavelength band is L-band, the first opticalamplifier is a C-band erbium doped fiber amplifier and the secondoptical amplifier is an L-band erbium doped fiber amplifier.
 5. Anoptical amplifier as claimed in claim 2, wherein the first wavelengthband is C-band and the second wavelength band is L-band.
 6. An opticalamplifier as claimed in claim 2, wherein the first wavelength band isC-band, the second wavelength band is L-band, the first opticalamplifier is a C-band erbium doped fiber amplifier and the secondoptical amplifier is an L-band erbium doped fiber amplifier.
 7. Anoptical amplifier as claimed in claim 2, wherein the controller controlsthe power of each pump light to reduce a wavelength characteristicdeviation of power between the first and second individual wavelengthbands.
 8. An optical amplifier as claimed in claim 2, wherein thecontroller controls the power of each pump light to set a power of eachfirst and second individual wavelength band to be equal.
 9. An opticalamplifier for amplifying a wavelength division multiplexed (WDM) lightincluding signal lights wavelength division multiplexed together, theamplifier comprising: an optical amplifying medium through which the WDMlight travels and is thereby amplified via Raman amplification inaccordance with multiplexed pump lights of different wavelengthstraveling through the optical amplifying medium; an optical devicedividing the amplified WDM light into first and second divided lights infirst and second wavelength bands, respectively; a first opticalamplifier amplifying the first divided light, the first opticalamplifier having a gain band including the first wavelength band; asecond optical amplifier amplifying the second divided light, the secondoptical amplifier having a gain band including the second wavelengthband; and a controller controlling output powers of the pump lights inaccordance with a difference in power of the first divided light beforeand after being amplified by the first optical amplifier, and with adifference in power of the second divided light before and after beingamplified by the second optical amplifier.
 10. An optical amplifier asclaimed in claim 9, wherein the first wavelength band is divided into aplurality of first individual wavelength bands, the second wavelengthband is divided into a plurality of second individual wavelength bands,and the controller controls output powers of the pump lights inaccordance with differences in power of the first divided light in eachfirst individual wavelength band before and after being amplified by thefirst optical amplifier, and in accordance with differences in power ofthe second divided light in each second individual wavelength bandbefore and after being amplified by the second optical amplifier.
 11. Anoptical amplifier as claimed in claim 9, wherein the first wavelengthband is C-band and the second wavelength band is L-band.
 12. An opticalamplifier as claimed in claim 9, wherein the first wavelength band isC-band, the second wavelength band is L-band, the first opticalamplifier is a C-band erbium doped fiber amplifier and the secondoptical amplifier is an L-band erbium doped fiber amplifier.
 13. Anoptical amplifier as claimed in claim 10, wherein the first wavelengthband is C-band and the second wavelength band is L-band.
 14. An opticalamplifier as claimed in claim 10, wherein the first wavelength band isC-band, the second wavelength band is L-band, the first opticalamplifier is a C-band erbium doped fiber amplifier and the secondoptical amplifier is an L-band erbium doped fiber amplifier.
 15. Anoptical amplifier as claimed in claim 10, wherein the controllercontrols the power of each pump light to reduce a wavelengthcharacteristic deviation of power between the first and secondindividual wavelength bands.
 16. An optical amplifier as claimed inclaim 10, wherein the controller controls the power of each pump lightto set a power of each first and second individual wavelength band to beequal.
 17. An optical amplifier for amplifying a wavelength divisionmultiplexed (WDM) light including signal lights wavelength divisionmultiplexed together, the amplifier comprising: an optical amplifyingmedium through which the WDM light travels and is thereby amplified viaRaman amplification in accordance with multiplexed pump lights ofdifferent wavelengths traveling through the optical amplifying medium;an optical device dividing the amplified WDM light into first and seconddivided lights in first and second wavelength bands, respectively; afirst optical amplifier amplifying the first divided light, the firstoptical amplifier having a gain band including the first wavelengthband; a second optical amplifier amplifying the second divided light,the second optical amplifier having a gain band including the secondwavelength band; and means for controlling power of each pump lightbased on a power of the first divided light amplified by the firstoptical amplifier and a power of the second divided light amplified bythe second optical amplifier.
 18. A method comprising: causing awavelength division multiplexed (WDM) light to travel through an opticalamplifying medium to thereby amplify the WDM light via Ramanamplification in accordance with multiplexed pump lights of differentwavelengths traveling through the optical amplifying medium, the WDMlight including signal lights wavelength division multiplexed together;dividing the amplified WDM light into first and second divided lights infirst and second wavelength bands, respectively; amplifying the firstdivided light with a first optical amplifier having a gain bandincluding the first wavelength band; amplifying the second divided lightwith a second optical amplifier having a gain band including the secondwavelength band; and controlling power of each pump light based on apower of the amplified first divided light and a power of the amplifiedsecond divided light.
 19. A method comprising: causing a wavelengthdivision multiplexed (WDM) light to travel through an optical amplifyingmedium to thereby amplify the WDM light via Raman amplification inaccordance with multiplexed pump lights of different wavelengthstraveling through the optical amplifying medium, the WDM light includingsignal lights wavelength division multiplexed together; dividing theamplified WDM light into first and second divided lights in first andsecond wavelength bands, respectively; amplifying the first dividedlight with a first optical amplifier having a gain band including thefirst wavelength band; amplifying the second divided light with a secondoptical amplifier having a gain band including the second wavelengthband; and controlling output powers of the pump lights in accordancewith a difference in power of the first divided light before and afterbeing amplified by the first optical amplifier, and a difference inpower of the second divided light before and after being amplified bythe second optical amplifier.